Manufacture of pitch



Patented July 6, 1954 MANUFACTURE OF PITCH Leslie Harrisville J uel,Arlington Heights, 111., as-

signor to Great Lakes Carbon Corporation, New York, N. Y., a corporationof Delaware No Drawing. Application October 5, 1951, Serial No. 250,015

Claims.

This invention relates to a process for manufacturing carbonaceouscompositions useful in the manufacture of carbon and graphiteelectrodes. More particularly, the invention relates to a process ofmanufacturing binders and impregnating agents from coal-derivedbitumens.

This application is a continuation-in-part of my co-pending applicationSer. No. 106,329, filed July 22, 1949 for Manufacture of Pitch, now

abandoned.

In the usual practice of manufacturing graphite electrodes which arecurrently employed in electrothermal and electronietallurgical hightemperature processes a carbonaceous aggregate, preferably petroleumcoke which has been calcined to less than 1% volatile matter, is admixedwith suitable amounts of a binder to form a composition which isextruded or molded. The resulting body is then baked and graphitized. Inorder to increase the strength and apparent density of the baked orgraphitized carbons they are often impregnated under vacuum with liquidcarbonaceous materials which are subsequently carbonized.

The most commonly used binder'and impregnant at the present time ispitch derived from coal tar. This pitch is a heterogeneous mixture ofcomplex hydrocarbons which may be broadly classified into about fourfractions; namely, a fraction insoluble in organic solvents andinsoluble in the pitch itself even when the latter is heated totemperatures up to about 400 C.; a fraction which is soluble in the coaltar (particularly when heated) but insoluble in lowboiling aromaticsolvents such as benzene; a fraction which is soluble in benzene butinsoluble in paraffimc hydrocarbons such as solvent naphtha, and afraction which is soluble in benzene and in solvent naphtha and lowboiling aliphatic compounds. It is known that certain fractions of coaltar or pitch derived therefrom are particularly suitable as binders forcarbon particles in 'b-the manufacture of carbon and graphiteelectrodes. Others have stated that the benzene-insoluble fractionexhibits poor binding or adhesive qualities with respect to the carbonaggregate. The solvent naphtha-soluble fraction exhibits some bindingcharacteristics but is somewhat objectionable in that it produces a lowcoke residue in the electrode baking operation. It has been pointed outthat the benzene-soluble fraction constitutes the mostadvantageouslfraction of coal tar or coal tar pitch fora binder for thecarbon particles in that it combines the advantages of high coke residuewith good adhesion to the particles.

Several investigators have proposed to prepare a carbonaceous binder forcarbon particles suitable in the manufacture of carbon electrodes fromcoal tar and pitches by employing a sequence of solvent extractionoperations whereby the benzene-soluble fraction is isolatedsubstantially free from the tar-insoluble, benzene-insoluble and solventnaphtha-soluble fractions. This is both time consuming and economicallyprohibitive from a commercial standpoint. It has also been proposed toprepare a carbonaceous binder for carbon particles by distilling coaltar or coal tar pitch, either at atmospheric pressure or in a vacuum, inorder to obtain a comparatively high boiling distillate which issubstantially soluble in benzene but which also contains substantialquantities of the undesirable solvent naphtha-soluble components whichproduce a low coke residue. The latter type of operation further suffersfrom the fact that up to 40% by weight of the original pitch or tar sodistilled remains in the still" as pitch coke and the yields of binderso produced are consequently not very high.

It is an object of this invention to provide a carbonaceous compositionsuitable as a binder for carbon aggregate in the manufacture of carbonelectrodes, particularly of the graphite type.

It is a further object of the invention to provide a process for themanufacture of carbonaceous binders and impregnating agents which aresubstantially free from matter undissolvable therein and which containsubstantial quantities the material contained therein which is insolubleand undissolvable in the bitumen even at elevated temperatures, such asup to 400 C. According to my invention, the removal of the aforesaidobjectionable material from the coal-derived bitumen is accomplishedwithout substantially affecting or reducing the benzene-insoluble orbenzene-soluble content of the bitumen. The treated bitumen is thenheated, if necessary, under controlled conditions to raise its meltingpoint and to increase the benzene-insoluble content without causingformation of material which is insoluble and undissolvable in thetreated bitumen.

In a broad embodiment the present invention 3 comprises removing from ahigh boiling liquefiable coal-derived bitumen matter which is insolubleand undissolvable therein even when the bitumen is heated for a timeinsufficient to cause substantial coking thereof. This is accomplishedin a manner such that the resulting product will be substantially freefrom an insoluble phase while retaining substantial amounts of benzene--insoluble matter which either was originally present in the material orwas formed during the treatment described herein. I have found that thepresence of the aforementioned undissolvable material in the bitumenswhich I employ has several disadvantages when such compositions are usedas a binder or impregnating agent for carbon particles in themanufacture of graphite electrodes. The presence of such material inelectrode binders and impregnants is believed to prevent the maximum anddesirable degree of penetration or impregnation of the carbon particlesby the more fluid components of the binder. Furthermore, the presence ofthis insoluble matter in the binder is detrimental to the formation ofbinder coke having a highly desirable structure during the electrodebaking operation.

The removal of the aforesaid undissolvable material from the highboiling liquefiable coalderived bitumen compositions may be conductedadvantageously by filtration and centrifugation processes, which will befurther herein described,

as contrasted with distillation techniques either at atmosphericpressure or at subatmospheric pressures. As previously explained herein,the distillation processes heretofore employed to produce liquid orliquefiable carbonaceous binders and impregna-nts from coal tar or pitchproduce materials which are substantially free from ben- Zens-insolublematerial. One of the important characteristics of the compositionsproduced in accordance with the present invention is the retentiontherein of a substantial amount of benzene-insoluble material whichnevertheless is soluble or dissolvable in the composition itself, andwhich will produce a high coke residue in the carbon electrode duringbaking.

ihe high boiling liquefiable bitumens with which this invention isparticularly concerned include coal derivatives such as retort and cokeoven tars and pitches, water gas tars and pitches,

straight run or cracked tars, and certain high.

boiling overheads produced from coal gasification processes. included inthe term pitch are the so-called soft, medium, and hard pitches having asoftening point of 75 to 125 C. Other materials which are adaptable tothe process of this invention are certain tar and pitch distillateswhich contain substantial amounts of benzeneinsoluble matter andmaterial undissolvable in the distillate itself, hydrogenated coal andcertain distillates therefrom; lignite, brown coal, and. tars andpitches therefrom. It has also been found that thermally produceddispersions of coal in oil which may or may not be subsequentlyhydrogenated, 'and certain distillates therefrom are useful inpracticing the invention. The bitumens should be essentiallyhydrocarbonaceone in nature and substantially free fromoxygen-containing compounds and inorganic matter. For the sake ofbrevity and convenience, these and related compositions are genericallyreferred to herein as high boiling, liquefiable coal-derived bitumens.The aforementioned materials, particularly in their crude state, arecharacterized by the presence of varying amounts of undissolvable matterwhich may be removed by filtration, centrifugation and equivalentmethods according tothis invention as contrasted with distillation ofsuch materials.

This invention does not contemplate the treatment of petroleum basematerials such as heavy residues or petroleum tars and pitches. Suchmaterials are deficient in aromatic, coke-producing constituents. Whilethey are substantially free from material which is insoluble andundissolvable in the main portion of the material they are also grosslydeficient in benzene-insoluble matter, the latter being an essentialcomponent in the novel compositions described herein.

I have found that the removal of substantially all of the aforementionedundissolvable material from the liquefiable coal-derived bitumens, whileretaining therein a substantial amount of benzenednsoluble matter whichis soluble in the liquefiable portion of the bitumen (especially uponapplication of heat), produces a material which is particularly suitableas a binder for carbon aggregate and as an impregnating agent formassive carbons in the manufacture of carbon and graphite electrodes.

In a preferred embodiment of the invention, coal tar which containsmatter insoluble and undissolvable therein even when the tar is heatedto 400 (3., is heated to a temperature between about 150 and 400 C. andthe liquefied material is filtered, preferably in conjunction with asmall quantity of diatomaceous earth filter aid. Usually between about0.5 and 6% by weight of filter aid will suffice. The liquid material isfed to a filtration apparatus, for example a plate and frame filterpress wherein the filtration medium such as canvas or heat-resistantmaterial contains a pre-coat of filter aid of the diatomaceous earthtype. The resulting filtrate is substantially free from theaforementioned undissolvable matter and will contain, after heattreatment, between about 15 and about 50% by Weight of hydrocarbonmaterial which is soluble or dissolvable in the filtrate itself but isinsoluble in benacne and similar low boiling solvents. The filtrate maythen be heated to temperatures preferably between about 400 and 525 C.and maintained in that temperature range until the material has asoftening point of between about and about 1 5 C. I have found that whenthe binders under consideration herein have a softening point aboveabout C. they are particularly desirable as binders for carbon aggregatein electrode manufacture.

In a further embodiment of the invention. coal tar or coal tar pitch isdiluted with a high boiling aromatic solvent such as a coal tardistillate or highly aromatic petroleum fractions having a boiling rangeof about 250 to 350 C. Generally speaking, any a-ro-matic solvent havinga boiling point or boiling range between about 200 andv about 400 C. maybe employed provided that the major (liquefiable) portion of the coalbitumen is soluble therein. Paramnic petroleum fractions such asso-called mineral or lubricatin oils are not suitable for thisoperation. Such materials precipitate or leave undissolved thebenzene-insoluble portion of the bitumen which is an essential elementof my novel compositions. In adding an aromatic solvent to a commercialcoal tar, a ratio of between about 20 and about 100 parts by weight ofsolvent to 100 parts by weight of tar has been found to be particularlysatisfactory. For a coal tar pitch, a ratio of between about one andabout five parts by weight of arcmatic solvent to one part by Weight ofpitch is advantageous. Dilution of the liquefiable coal bitumen withsolvents of the aforementioned type decreases the viscosity of thecompositions'and facilitates filtration. The extent of dilution shouldnot exceed the point where substantial quantities of the desirablebenzene-insoluble fraction of the coal bitumen will precipitate from themixture. It is one of the main objects of the present invention toremove the bitumen-undissolvable matter from the bitumen while retainingsubstantial quantities of the benzene insoluble fraction in the finalcomposition.

In the event that a solvent is employed in the filtration operation, thefiltrate is preferably preliminarily heated to distill and recover thesolvent followed by continued heating to produce a binder orimpregnating agent having a melting point in the preferred range. I

Removal of the objectionable undissolvable matter may also beaccomplished by filtration on a rotary filter of the Oliver type, thelatter containing a canvas or other suitable medium preferablypre-coated with a filter aid of the type previously mentioned herein aswell as other suitable filter aids.

The undissolvable matter may also be removed by centrifugationoperations, preferably subsequent to dilution of the coal bitumen inaccordance with the above mentioned procedures.

In a further embodiment of the invention, I may add to the novel bindersdescribed herein certain nitro and/or halogen aromatic compounds such asare described in Patents Nos. 2,500,208 and 2,500,209. These compoundsmay be added to the coal-derived bitumen prior to the removal of theundissolvable matter therefrom in accordance with techniques describedherein provided that the following operating temperatures do not exceedthe temperature at which the compound reacts with the bitumen.

Alternatively, nitro or halogen aromatic compounds may be added to thebinder subsequent to removal of undissolvable matter therefrom toproduce a liquefiable single-phase material. Alternatively, theaforementioned compounds may be added to the binder during the mixingoperation whereby the binder and carbon particles are mixed at atemperature of about 150 C. These compounds may be advantageouslyincorporated into the binder of the present invention in F amounts up toabout 45% by weight of the binder when such compound are utilized toincrease the coke residue of the binder. When the main object of theaddition of such compounds is to take advantage of their lubricatingqualities, the amount added to the binder is between about 2 and about5% by weight, further provided that a portion of the added compoundremains in soluble in the binder itself during the mixing operation.

In producing a binder or impregnating agent for electrodes in accordancewith the present invention, it is desirable that the composition havebetween about 15 and 50% by weight of benzene-insoluble material(Barrett test No. B7) which, however, will be soluble or dissolvable inthe binder itself. Small quantities of certain components ofthese-binders may precipitate therefrom upon prolonged standing at roomtemperature, but these may readily be dissolved in the binder by heatingor will become dissolved at temperatures employed during the mixingoperation wherein the binder and carbon particles are admixed prior tothe formation of green electrodes.

It has been found that graphite electrodes produced from a carbonaggregate and the present novel binders have improvements in electricalresistivities of 10% or more in comparison to graphite electrodesproduced using the conventional coal tar pitch binders. It should beemphasiz ed that the lowering of resistivity values in graphite"electrodes even to a minor degree is particularly desirable andadvantageous to the users of such electrodes. One of the main coats inoperating an electrothermal or electrometallurgical high temperaturefurnace is the electric power consumption and the 10% or more reductionin electrical resistivity of my graphite electrodesis therefore highlydesirable.

To more fully illustrate the nature and character of the inventionbutwith no intention of being limited thereby, the following examplesare set forth:

Example I One hundred parts by weight of a commercial coal tarwhichcontains about 14 to 15% of filterable solid phase material(undissolvable upon heating the tar to a temperature of 300 C.) wasdiluted with about parts by weight of a high boiling coal 'tardistillate (boiling range 230 to 310 C., specific gravity 1.0-1.04).This mixture was heated to a temperature of about 200 C. To theliquefied material was added about 1% by weight of a high flow ratediatomaceous earth filter aid and the resulting mixture was filteredthrough a filter press containing a precoat of diatomaceous earth filteraid. The resulting filtrate was then heated to recover the solvent andheating was further continued at a temperature between about 400 to 500C. until the material had a softening point of 86 C. (ball and ring).This'material was substantially free from undissolvable matter and had abenzene-insoluble content of about 24%. It was used in the preparationof graphite electrodes as follows:

A calcined petroleum coke flour consisting of :2% of 48/200 mesh and theremainder minus 200 mesh was employed as the carbon aggregate. To partsby weight of flour was admixed 36 parts by weight of the binder preparedin the above described'manner and the mixing operation was conducted atabout C. The resulting mass was extruded through a die at a pressure ofabout 5,000 pounds per square inch at a temperature of about 90 to forma "green electrode. The latter was baked to a temperature of 1,000 C.and thengraphitized to a temperature of 2900 C. The resulting electrodehad a longitudinal electrical resistivity ohms/in. of 29.2 10- Agraphite electrode prepared according to the above described processfrom petroleum coke flour and a commercially available coal tar pitchhaving a softening point of 85 C. and a benzene insoluble content of23.5% had an electrical resistivity value of 33.8 10 It can be seen thata 13.6% reduction or improvement in electrical resistivity is obtainableby employing the carbonaceous binders of the present invention ascompared to electrodes produced from standard coal tar pitch binders.

Example II 109 C. and a benzene insoluble content of 28.5% by weight.When liquefied, the binder was substantially free from an insolublephase.

An electrode mix was prepared by adding 36 parts by weight of thisbinder to 100 parts of calcined petroleum coke flour of the typeemployed in Example I and a graphite electrode was prepared as describedin Example I. This electrode had an electrical resistivity value of 28.210 ohms/in? which is a 16.5% reduction of improvement in resistivitycompared to graphite electrodes prepared with conventional coal tarpitch binders.

Example III An electrode was prepared from calcined petroleum cokeemploying 60 parts by weight of coke particles having a particle sizedistribution of l to 1 to 1 of 8-10: -14: i i-20 mesh, and 40 parts ofcoke fiouroi the type employed in EX- ample I. To 100 parts by weight ofthis mixture was added 27 parts by weight of a coal tar-derived binderprepared as described in Example I. A mixing operation was conducted atabout 180 C. A graphite electrode formed from this mixture in accordancewith the procedure of Example I had electrical resistivity value of 10 X10- ohms/ind.

To 100 parts by weight of the calcined petroleum coke mixture describedabove were added 27 parts by weight of a conventional coal tar pitchbinder. Mixing was conducted at a temperature of about 160 C. and agraphite electrode was prepared from the resulting mixture in accordancewith the procedure described in Example I. This electrode had anelectrical resistivity of 47.9X10 ohms/in. which is 19.7% higher thanthe electrode prepared with the novel binder used above.

' Example IV One hundred parts by weight of a commercial coal tar pitch(20% filterablesolid phase) was heated to a temperature of 160 C. anddiluted with 200 parts by weight of a high boiling coal tar distillate.After heating the mixture to about 200 C. it was centrifuged toconcentrate the insoluble phase. The substantially solid-free solublephase was heated to remove the solvent and heating was further continuedat a temperature between 400 to 500 C. until the resulting material hada softening point of 95 C. This material had a benzen -insoluble contentof 26% by weight and was free from undissolvable material (materialwhich would not dissolve upon heating the binder to 250 C.).

A graphite electrode was prepared according to the procedure of Example11, and a similar improvement respecting electrical. resistivity wasobserved.

While the ioreogoing examples particularly illustrate the use of coaltars and pitches as the source of the carbonaceous binders of thepresent invention and describe the preparation of said binders byparticular types of filtration operation, it is to be understood thatother compositions of the type herein described and which are within thescope of the expression high boiling coal-derived liquefiable bitumensmay be employed without departing from. the spirit of the invention.Furthermore, it is to be understood that other types of operations forthe removal of the matter undissolvable in such compositions attemperatures below their coking temperatures may be employed providingsuch operations reti tain substantial amounts of benzene-insolublematerial in the resulting binders.

I claim as my invention:

1. A composition suitable as a binder for carbon particles and as animp-regnant in the manufacture of graphite electrodes comprisingessentially coal tar pitch substantially free from matter insolubletherein at a temperature between about 100 and about 400 0., saidcomposition containing between about 15 and about 50% by weight ofbenzene-insoluble matter which is soluble therein and having a softeningpoint of between about and about 125 C.

2. A process for producing a composition suitable as a binder for carbonparticles and as an impregnant in the manufacture of graphite electrodesfrom coal tar which contains matter substantially insoluble therein at atemperature between about and about 400 C. which comprises filteringsaid tar in conjunction with a, filter aid at a temperature betweenabout 150\ and 400 C. to produce a filtrate substantially free fromundissolvable matter and heating the resulting filtrate to a temperaturebetween about 100 and 525 C. until the material has a softening pointbetween about 85 and about C. and contains between about 15 and about50% by weight of benzene-insoluble matter which is soluble in saidmaterial.

3. A process according to claim 2, wherein a high boiling aromaticdiluent having a boiling range between about 200 and about 400 C. isadded to the tar prior to filtration in an amount between about 20 andabout 100 by weight of said diluent per 100 parts by Weight of tar, saiddiluent being a solvent for the benezene-insoluble tar fraction but anon-solvent for said undissclvable matter.

4. A process for producing a composition suitable as a binder for carbonparticles and as an impregnant in the manufacture of graphite electrodesfrom coal tar pitch which contains matter substantially insolubletherein at a temperature between about 100 and about i00 0., whichcomprises filtering said pitch in conjunction with a filter aid at atemperature between about 200 and about 100 C. to produce a filtratesubstantially free from insoluble matter but which contains betweenabout 15 and about 50% by weight of benzene-insoluble matter which issoluble in said filtrate.

5. A process according to claim 1 wherein a high boiling aromaticdiluent having a boiling range between about 200 and about 400 C. isadded to the pitch prior to filtration in an amount between about 1 toabout 5 parts by weight of solvent to 1 part by weight of pitch, saiddiluent being a solvent for the benzene-insoluble pitch fraction but anon-solvent for said undissolvable matter.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 951,993 Lowry et a1 Mar. 15, 1910 1,355,099 Weiss Oct. 5, 19201,708,602 Black et al Apr. 0, 1929 2,024,106 Levin Dec. 10, 19352,297,455 Brautigam et a1. Sept. 29, 1942 2,340,847 Parker Feb. l, 19442,395,853 Fair Mar. 5, 1946 2,549,298 Donigan Apr. 17, 1951

1. A COMPOSITION SUITABLE AS A BINDER FOR CARBON PARTICLES AND AS ANIMPREGNANT IN THE MANUFACTURE OF GRAPHITE ELECTRODES COMPRISINGESSENTIALLY COAL TAR PITCH SUBSTANTIALLY FREE FROM MATTER INSOLUBLETHEREIN AT A TEMPERATURE BETWEEN ABOUT 100* AND ABOUT 400* C., SAIDCOMPOSITION CONTAINING BETWEEN ABOUT 15 AND ABOUT 50% BY WEIGHT OFBENZENE-INSOLUBLE MATTER WHICH IS SOLUBLE THEREIN AND HAVING A SOFTENINGPOINT OF BETWEEN ABOUT 85% AND ABOUT 125* C.
 2. A PROCESS FOR PRODUCINGA COMPOSITION SUITABLE AS A BINDER FOR CARBON PARTICLES AND AS ANIMPREGNANT IN THE MANUFACTURE OF GRAPHITE ELECTRODES FROM COAL TAR WHICHCONTAINS MATTER SUBSTANTIALLY INSOLUBLE THEREIN AT A TEMPERATURE BETWEENABOUT 100* AND ABOUT 400* C. WHICH COMPRISES FILTERING SAID TAR INCONJUNCTION WITH A FILTER AID AT A TEMPERATURE BETWEEN ABOUT 150* AND400* C. TO PRODUCE A FILTERATE SUBSTANTIALLY FREE FROM UNDISSOLVABLEMATTER AND HEATING THE RESULTING FILTRATE TO A TEMPERATURE BETWEEN ABOUT400* AND 525* C. UNTIL THE MATERIAL HAS A SOFTENING POINT BETWEEN 85*AND ABOUT 125* C. AND CONTAINS BETWEEN ABOUT 15 AND ABOUT 50% BY WEIGHTOF BENZENE-INSOLUBLE MATTER WHICH IS SOLUBLE IN SAID MATERIAL.